[Schematic Capillary Pressure]

[Starling's Relationship]

As you move your cursor along the capillary from the arteriolar end to the venule end you will see the resultant pressure gradient at that point in the capillary and be able to relate that to net flow of fluid in or out of the capillary.

[Vasodilation]

Vasodilation reduces the pressure drop across the arterioles, so that capillary pressure is closer to the arterial pressure. The venous pressure may remain unchanged. The net result is a greater length of capillary where fluid leaves the plasma, and a reduced length for it to return. 

This imbalance results in a net loss of fluid from the plasma. The result is an increase of the interstitial fluid in this tissue. If this increase continued, it would result in edema.

[Shock]

When the central blood pressure decreases, the pressure at the capillaries usually decreases. Most vascular beds will initiate reflex efforts to maintain the central blood pressure via arteriolar vasoconstriction. This further reduces the pressure at the arteriolar end of the capillary.

The decrease in hydrostatic pressure results in a decreased length of capillary where fluid is lost from the plasma, and an increase in the length where fluid returns. This results in a net gain of fluid to the plasma. If this occurs over most of the body's vascular beds there is an "auto-transfusion" that helps to compensate for plasma loss during hemorrhagic shock.

[Congestive Heart Failure]

When cardiac function is compromised, it cannot pump blood as effectively, so venous pressure rises. This elevation in venous pressure is seen in the capillaries. This rise in venous pressure decreases the length of capillary where fluid is reabsorbed into the plasma. This results in a net loss of fluid from plasma to ISF. The resulting edema can be seen in the swollen ankles (and other tissues) that are symptomatic of congestive heart failure.

[Decreased Oncotic Pressure]

When the plasma does not contain sufficient protein, or the interstitial fluids contain too much, then the difference in oncotic pressures falls from 25 mm Hg to less than 20. This results in a net increase in the length of the capillary where hydrostatic pressure exceeds oncotic pressure, which leads to edema. This imbalance explains the kwashiorkor appearance of children with severe protein malnourishment from peritoneal edema. In burns, when capillaries are damaged, more plasma proteins leak into the interstitium, resulting in a decrease of the oncotic pressure difference, and edema.